Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Aug 11, 2020 at 10:19:17AM +0200, Michal Hocko wrote:
> On Mon 10-08-20 21:25:26, Michal Hocko wrote:
> > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> [...]
> > > The problem that i see is we can not use the page allocator from atomic
> > > contexts, what is our case:
> > > 
> > > <snip>
> > >     local_irq_save(flags) or preempt_disable() or raw_spinlock();
> > >     __get_free_page(GFP_ATOMIC);
> > > <snip>
> > > 
> > > So if we can convert the page allocator to raw_* lock it will be appreciated,
> > > at least from our side, IMHO, not from RT one. But as i stated above we need
> > > to sort raised questions out if converting is done.
> > > 
> > > What is your view?
> > 
> > To me it would make more sense to support atomic allocations also for
> > the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
> > work for atomic context in RT sounds subtle and wrong.
> 
> I was thinking about this some more. I still think the above would be a
> reasonable goal we should try to achieve. If for not other then for
> future maintainability (especially after the RT patchset is merged).
> I have tried to search for any known problems/attempts to make
> zone->lock raw but couldn't find anything. Maybe somebody more involved
> in RT world have something to say about that.
> 
I tried yesterday to convert zone->lock. See below files i had to modify:
<snip>
        modified:   include/linux/mmzone.h
        modified:   mm/compaction.c
        modified:   mm/memory_hotplug.c
        modified:   mm/page_alloc.c
        modified:   mm/page_isolation.c
        modified:   mm/page_reporting.c
        modified:   mm/shuffle.c
        modified:   mm/vmscan.c
        modified:   mm/vmstat.c
<snip>

There is one more lock, that is zone->lru_lock one. Both zone->lock and this
one intersect between each other. If the lru_lock can be nested under zone->lock
it should be converted as well. But i need to analyze it farther. There are
two wrapper functions which are used as common interface to lock/unlock both
locks. See compact_lock_irqsave()/compact_unlock_should_abort_lru() in the 
mm/compaction.c.

Any thoughts here?

Anyway i tried to convert only zone->lock and use page allocator passing there
gfp_mask=0 as argument. So it works. CONFIG_PROVE_RAW_LOCK_NESTING does not
complain about any "bad" lock nesting.

> Anyway, if the zone->lock is not a good fit for raw_spin_lock then the
> only way I can see forward is to detect real (RT) atomic contexts and
> bail out early before taking the lock in the allocator for NOWAIT/ATOMIC
> requests.
>
For RT kernel we can detect it for sure. preemtable() works just fine there,
i.e. we can identify the context we are currently in.

Thanks!

--
Vlad Rezki